target = 9
a = [15,7,11,2]

# 定义一个链表
class Node:
    # ->用于提示函数的返回类型
    def __init__(self,val) -> None:
        self.val = val
        self.next = next

class Slink:
    def __init__(self) -> None:
        self.head = 

for i in range(0,4):
    for j in range(i+1,4):
        sum = a[i] + a[j]
        if sum is target:
            print("{},{}".format(i,j))
            break

        
